NO.sub.x in the atmosphere is the cause of photochemical smog and acid rain. Hence, the discharge of NO.sub.x from moving sources like automobiles incorporating internal combustion engines, such as gasoline engine and diesel engine, poses a social problem. The internal combustion engine is one of the NO.sub.x sources. Therefore, there is a tendency towards tightening up the laws and regulations concerning the amount of discharge of NO.sub.x. Accordingly, the development of exhaust emission control catalysts is being actively carried out.
As a conventional exhaust emission control catalyst for controlling the emission of exhaust from the gasoline engine, a so-called three-way catalyst capable of simultaneously reducing NO.sub.x, unburned hydrocarbon, and carbon monoxide is known. Since the exhaust from a typical gasoline engine contains very little oxygen, it is possible to achieve efficient reduction of NO.sub.x by unburned hydrocarbon or carbon monoxide, and decrease NO.sub.x.
However, the exhaust from the diesel engine contains excessive oxygen because of its engine characteristics. Moreover, stoichiometrically, the exhaust from the diesel engine contains less hydrocarbon and carbon monoxide which function as a reducing agent compared to NO.sub.x. Therefore, when a conventional three-way catalyst is used for the treatment of the exhaust from the diesel engine, NO.sub.x can hardly be decreased.
Furthermore, since the exhaust from the diesel engine contains a large amount of particulate matter formed by carbons, soluble organic fractions (SOF), sulfate, etc., the amounts of these elements discharged are restricted by the laws and regulations. Therefore, when using a typical three-way catalyst for the treatment of the exhaust from the diesel engine, it is also required to reduce the particulate matter. However, such a catalyst can hardly decrease the particulate matter.
In resent years, a lean burn gasoline engine, and a cylinder injection of fuel type gasoline engine have been developed for the purpose of decreasing the fuel consumption. Since these engines cause lean burn, the oxygen concentration in the exhaust from these engines is high. Therefore, when a typical three-way catalyst is used for the treatment of the exhaust from such gasoline engines, it is difficult to decrease NO.sub.x.
In order to solve this problem, a catalyst containing a porous carrier like zeolite carrying copper is proposed as an exhaust emission control catalyst which effectively removes NO.sub.x in exhaust containing a large amount of oxygen, such as exhaust from the diesel engine and exhaust from the lean burn gasoline engine. An example of such a catalyst is disclosed in Japanese Publication for Unexamined Patent Application No. 100919/1988 (Tokukaisho 63-100919). However, this catalyst is inferior in heat resistance, and its NO.sub.x removing ability tends to be lowered by sulfur oxides like SO.sub.2 contained in the exhaust. Namely, there is a problem that the catalyst is readily poisoned.
Moreover, Japanese Publication for Unexamined Patent Application No. 137963/1993 (Tokukaihei 5-137963) discloses an exhaust emission control catalyst containing platinum as a principal component. However, since this catalyst is highly active to oxidize SO.sub.2 in the exhaust, a large amount of sulfates are produced by the oxidation of SO.sub.2, resulting in an increase in the content of sulfates in the exhaust. Thus, there is a problem that this catalyst increases the amount of particulate matter in the exhaust due to the sulfates. In particular, in the case of the exhaust from a diesel engine which contains a larger amount of particulate matter compared to a gasoline engine, it is required to reduce the discharge of the particulate matter to a lower level. Hence, there is a more serious problem with respect to the exhaust from the diesel engine, namely an increased amount of particulate matter is produced.
Furthermore, Japanese Publication for Unexamined Patent Application No. 219147/1992 (Tokukaihei 4-219147) discloses an exhaust emission control catalyst containing a particular zeolite which carries cobalt, copper and/or rhodium, and rare earth metal as essential components.
However, only an exhaust emission control catalyst using lanthanum or cerium as the rare earth metal is disclosed as an example in this publication. When lanthanum or cerium is used as the rare earth metal, the activity of oxidizing SO.sub.2 in the exhaust becomes higher. Therefore, when this catalyst is used for the treatment of high temperature exhaust from a diesel engine, a large amount of sulfates are produced by the oxidation of SO.sub.2, resulting in an increase in the content of the sulfates in the exhaust. Thus, like the above-mentioned catalyst, this catalyst causes a problem that the amount of particulate matter in the exhaust is increased.
Considering the above-mentioned conventional problems, it is an object of the present invention to provide exhaust emission control catalysts and a method for controlling the emission of exhaust, which can efficiently decrease NO.sub.x in exhaust containing a large amount of oxygen like exhaust from a diesel engine, and reduce the amount of particulate matter in the exhaust under high temperature conditions.